1 00:00:00,000 --> 00:00:00,016 The following content is provided under a Creative 2 00:00:00,016 --> 00:00:00,022 Commons license. 3 00:00:00,022 --> 00:00:00,038 Your support will help MIT OpenCourseWare continue to 4 00:00:00,038 --> 00:00:00,054 offer high quality educational resources for free. 5 00:00:00,054 --> 00:00:00,072 To make a donation or view additional materials from 6 00:00:00,072 --> 00:00:00,088 hundreds of MIT courses, visit MIT OpenCourseWare at 7 00:00:00,088 --> 00:00:00,110 ocw.mit.edu. 8 00:00:00,110 --> 00:00:23,530 PROFESSOR: OK, it's time to get started. 9 00:00:23,530 --> 00:00:25,590 Pay attention to the clicker questions. 10 00:00:25,590 --> 00:00:59,520 All right, I'll give you 10 more seconds. 11 00:00:59,520 --> 00:01:09,680 Not bad, we seem to be in the 70's. 12 00:01:09,680 --> 00:01:10,220 All right. 13 00:01:10,220 --> 00:01:14,380 So if you didn't have time to click in, let's consider 14 00:01:14,380 --> 00:01:18,270 what's going on here. 15 00:01:18,270 --> 00:01:28,390 What's true about the relationship of q and k here? 16 00:01:28,390 --> 00:01:31,940 So is q less than or greater than k? 17 00:01:31,940 --> 00:01:34,810 Less than. 18 00:01:34,810 --> 00:01:38,070 And so you have to think about what that means in terms of 19 00:01:38,070 --> 00:01:40,560 where we are now in the reaction and where the 20 00:01:40,560 --> 00:01:43,550 reaction is at equilibrium, and I like to think about it 21 00:01:43,550 --> 00:01:46,610 in terms of products -- are there more products now, or 22 00:01:46,610 --> 00:01:48,980 are there more products at equilibrium. 23 00:01:48,980 --> 00:01:52,850 So if q is less than k, then there are more products at 24 00:01:52,850 --> 00:01:55,630 equilibrium, and so the reaction will 25 00:01:55,630 --> 00:01:58,300 shift toward products. 26 00:01:58,300 --> 00:02:01,090 So it's going to move in the direction to reach 27 00:02:01,090 --> 00:02:05,220 equilibrium, since k is a bigger number than q, and both 28 00:02:05,220 --> 00:02:08,360 of these terms are products over reactants, there'd mean 29 00:02:08,360 --> 00:02:10,620 there's more product at equilibrium then there are 30 00:02:10,620 --> 00:02:12,570 now, and so the reaction is going to 31 00:02:12,570 --> 00:02:14,300 shift toward products. 32 00:02:14,300 --> 00:02:17,300 All right, so we're at 73%. 33 00:02:17,300 --> 00:02:21,760 Let's see if by the end of today, we can get up to 90 on 34 00:02:21,760 --> 00:02:22,860 these kinds of questions. 35 00:02:22,860 --> 00:02:29,680 All right, so we're going to continue to consider external 36 00:02:29,680 --> 00:02:32,980 effects on k in today's class. 37 00:02:32,980 --> 00:02:38,060 I also wanted to mention that on each of the handouts for 38 00:02:38,060 --> 00:02:42,380 the lectures, I have the corresponding reading material 39 00:02:42,380 --> 00:02:46,460 listed on the handout, and I've listed it as section 40 00:02:46,460 --> 00:02:49,970 numbers in the chapters rather than page numbers, so that's a 41 00:02:49,970 --> 00:02:51,330 little bit of a difference. 42 00:02:51,330 --> 00:02:54,710 And the reason why I've done that is that we've had many 43 00:02:54,710 --> 00:02:58,140 different versions of the book in this class, and they don't 44 00:02:58,140 --> 00:03:02,890 seem to change the section titles or the section numbers, 45 00:03:02,890 --> 00:03:06,330 but they do change the page numbers associated with them. 46 00:03:06,330 --> 00:03:08,720 So if I give you section numbers then you should be 47 00:03:08,720 --> 00:03:11,410 able to use of whichever version of the book is 48 00:03:11,410 --> 00:03:12,800 available to you. 49 00:03:12,800 --> 00:03:17,070 So that's how I have it listed now, and the reading 50 00:03:17,070 --> 00:03:21,240 assignment is also listed on each problem-set as well. 51 00:03:21,240 --> 00:03:24,080 So when you're going over the handouts later studying for 52 00:03:24,080 --> 00:03:28,990 the exam, you can see where in the book this particular 53 00:03:28,990 --> 00:03:31,390 lecture, the material is, so you can 54 00:03:31,390 --> 00:03:32,820 go and do that reading. 55 00:03:32,820 --> 00:03:36,810 All right, so we talked at the end of the class last time 56 00:03:36,810 --> 00:03:42,030 about the Le Chatelier's principle, and this is a 57 00:03:42,030 --> 00:03:45,200 principle that you can use to predict the direction of 58 00:03:45,200 --> 00:03:47,250 change of a reaction -- whether it'll shift to the 59 00:03:47,250 --> 00:03:49,240 right or shift to the left. 60 00:03:49,240 --> 00:03:53,810 And simply stated, that systems tend to respond in 61 00:03:53,810 --> 00:03:56,260 such a way to minimize the stress. 62 00:03:56,260 --> 00:03:58,940 So if something is added to the reaction, the reaction 63 00:03:58,940 --> 00:04:03,090 will shift in a way to minimize that stress caused by 64 00:04:03,090 --> 00:04:06,390 the thing that's added to the system. 65 00:04:06,390 --> 00:04:11,440 So, we talked about adding reagents and removing products 66 00:04:11,440 --> 00:04:15,850 last time, and now we're going to go on and talk about what 67 00:04:15,850 --> 00:04:18,760 happens when you change the volume. 68 00:04:18,760 --> 00:04:21,220 So if you have a system at equilibrium and you change the 69 00:04:21,220 --> 00:04:24,060 volume of it, and here in particular we're talking about 70 00:04:24,060 --> 00:04:28,460 a gaseous system, what's going to happen? 71 00:04:28,460 --> 00:04:33,370 So what do we know about volumes 72 00:04:33,370 --> 00:04:35,470 when it comes to gases? 73 00:04:35,470 --> 00:04:38,350 We say that a decrease in volume of the gaseous system 74 00:04:38,350 --> 00:04:41,060 causes an increase in total pressure. 75 00:04:41,060 --> 00:04:48,360 What equation leaps to your mind when I say those words? 76 00:04:48,360 --> 00:04:53,110 Exactly, p v equals n r t. 77 00:04:53,110 --> 00:04:55,960 Yes. 78 00:04:55,960 --> 00:04:59,980 So there's a relationship between pressure and volume -- 79 00:04:59,980 --> 00:05:04,880 n is the number of moles, r is our gas constant, and t is our 80 00:05:04,880 --> 00:05:05,720 temperature. 81 00:05:05,720 --> 00:05:09,360 This is one of the things that usually sticks from high 82 00:05:09,360 --> 00:05:13,140 school, and if you haven't seen it before, it's pretty 83 00:05:13,140 --> 00:05:18,130 easy to get up to speed with this equation. 84 00:05:18,130 --> 00:05:21,900 So Le Chatelier's principle then predicts that the system 85 00:05:21,900 --> 00:05:25,860 will respond if possible in a way to 86 00:05:25,860 --> 00:05:27,400 reduce the total pressure. 87 00:05:27,400 --> 00:05:30,660 So if you decrease the volume and you cause an increase in 88 00:05:30,660 --> 00:05:33,820 total pressure, Le Chatelier says wait a minute, the system 89 00:05:33,820 --> 00:05:37,080 wants to respond to minimize that stress, so that it's 90 00:05:37,080 --> 00:05:41,460 going to respond in such a way to reduce that total pressure. 91 00:05:41,460 --> 00:05:44,620 So let's look at some examples of this. 92 00:05:44,620 --> 00:05:48,710 So we have an example of a reaction where we have two 93 00:05:48,710 --> 00:05:52,520 moles of p 2 gas going to one mole going to p 4 gas. 94 00:05:52,520 --> 00:05:56,160 And I have a little cartoon here to show this. 95 00:05:56,160 --> 00:06:02,670 So we have our p 4 gas up here, and our p 2 gas. 96 00:06:02,670 --> 00:06:04,750 All right, so what's going to happen if we're going to 97 00:06:04,750 --> 00:06:07,560 change the volume of this system? 98 00:06:07,560 --> 00:06:13,440 So if we're going to decrease the volume of the system, then 99 00:06:13,440 --> 00:06:15,470 the reaction should shift toward products. 100 00:06:15,470 --> 00:06:16,910 So let's think about that. 101 00:06:16,910 --> 00:06:26,040 So here in the middle we have our sort of system, and then 102 00:06:26,040 --> 00:06:30,340 on one side what happens when you decrease the volume and on 103 00:06:30,340 --> 00:06:32,370 the other side we're expanding the volume. 104 00:06:32,370 --> 00:06:35,010 So first, let's consider our system when we're going to 105 00:06:35,010 --> 00:06:39,740 compress that volume, decrease that volume. 106 00:06:39,740 --> 00:06:42,780 So what would be true is that the system's going to respond 107 00:06:42,780 --> 00:06:47,080 in such a way to minimize that stress that has been put 108 00:06:47,080 --> 00:06:49,840 forward by decreasing the volume, and you're going to 109 00:06:49,840 --> 00:06:56,200 try to compensate for that, and in this case you can 110 00:06:56,200 --> 00:06:58,210 compensate by a shift toward products. 111 00:06:58,210 --> 00:06:58,880 Why is that? 112 00:06:58,880 --> 00:07:01,500 Well, it all has to do with the stoichiometry of this 113 00:07:01,500 --> 00:07:02,820 equation up here. 114 00:07:02,820 --> 00:07:05,700 So for every two moles of reactants, you 115 00:07:05,700 --> 00:07:08,960 get one mole of products. 116 00:07:08,960 --> 00:07:14,570 So if you shift from the two to the one, that will cause a 117 00:07:14,570 --> 00:07:18,560 decrease in pressure, and so that will help compensate for 118 00:07:18,560 --> 00:07:20,450 the increase in pressure caused by 119 00:07:20,450 --> 00:07:21,930 the switch in volume. 120 00:07:21,930 --> 00:07:24,920 So you're going to respond in such a way to compensate for 121 00:07:24,920 --> 00:07:26,310 that stress. 122 00:07:26,310 --> 00:07:28,440 So a shift to the reaction to the right 123 00:07:28,440 --> 00:07:31,030 decreases the total pressure. 124 00:07:31,030 --> 00:07:33,560 So now let's think more about why this is true. 125 00:07:33,560 --> 00:07:36,590 Let's think about it from a math perspective and also just 126 00:07:36,590 --> 00:07:40,870 this qualitative perspective. 127 00:07:40,870 --> 00:07:45,570 So let's consider it in terms of q and k. 128 00:07:45,570 --> 00:07:49,120 So suppose our volume is decreased by a factor of 2, 129 00:07:49,120 --> 00:07:51,170 let's just make it easy, and we have 130 00:07:51,170 --> 00:07:54,030 constant temperature here. 131 00:07:54,030 --> 00:07:58,000 So this change will increase the partial pressure of both 132 00:07:58,000 --> 00:08:01,520 gases, the reacting gas and the product gas, and it's 133 00:08:01,520 --> 00:08:02,920 going to increase them both by the same 134 00:08:02,920 --> 00:08:05,010 amount, by 2 initially. 135 00:08:05,010 --> 00:08:09,060 So let's look at our equation for q, the reaction quotient. 136 00:08:09,060 --> 00:08:12,620 We have partial pressure of products over the partial 137 00:08:12,620 --> 00:08:16,260 pressures of the reactants, a raise to the coefficient to 138 00:08:16,260 --> 00:08:17,660 the equation. 139 00:08:17,660 --> 00:08:21,710 So initially you're going to increase the pressure, partial 140 00:08:21,710 --> 00:08:25,110 pressure of both gases by 2 -- then your product by 2, the 141 00:08:25,110 --> 00:08:28,970 reactant by 2 -- but here it's 2 squared. 142 00:08:28,970 --> 00:08:33,080 So overall, q becomes 1/2. 143 00:08:33,080 --> 00:08:36,810 So q decreases by a factor of 2, and so q 144 00:08:36,810 --> 00:08:39,570 becomes less then k. 145 00:08:39,570 --> 00:08:41,850 And so, in the clicker question, we talked about what 146 00:08:41,850 --> 00:08:47,640 happens when q becomes less than k, and what happens there 147 00:08:47,640 --> 00:08:50,930 is you're going to shift toward products 148 00:08:50,930 --> 00:08:54,360 until q equals k again. 149 00:08:54,360 --> 00:08:57,000 So you can think about this in terms of q and k, and you can 150 00:08:57,000 --> 00:08:59,990 also just think about it how many moles of gas are on one 151 00:08:59,990 --> 00:09:03,320 side, how many mold of gas on the other side, and how are 152 00:09:03,320 --> 00:09:06,070 you going to correspond, how are you going to decrease or 153 00:09:06,070 --> 00:09:08,410 minimize the stress on the system. 154 00:09:08,410 --> 00:09:14,290 All right, so let's think about what happens if we 155 00:09:14,290 --> 00:09:17,390 increase the volume, so here is our system and now we're 156 00:09:17,390 --> 00:09:19,690 going to expand it. 157 00:09:19,690 --> 00:09:22,760 If we're going to increase the volume, what's going to happen 158 00:09:22,760 --> 00:09:30,920 to the total pressure? 159 00:09:30,920 --> 00:09:33,850 I'm hearing it, but I want everyone to answer. 160 00:09:33,850 --> 00:09:36,410 Thank you. 161 00:09:36,410 --> 00:09:38,970 All right, so it will decrease the pressure. 162 00:09:38,970 --> 00:09:43,310 And what that's going to do is shift toward reactants, 163 00:09:43,310 --> 00:09:46,570 because we want to compensate for that decrease in pressure, 164 00:09:46,570 --> 00:09:49,590 and to do that, we can increase the pressure by 165 00:09:49,590 --> 00:09:53,550 switching or shifting the reaction toward the reactants. 166 00:09:53,550 --> 00:09:58,350 So again, we're going from one mole of product on one side to 167 00:09:58,350 --> 00:10:00,930 two of the reactants. 168 00:10:00,930 --> 00:10:06,840 So here, we see a shift toward the reactants. 169 00:10:06,840 --> 00:10:11,140 And so, this shift to the left is going to increase the total 170 00:10:11,140 --> 00:10:15,300 pressure to compensate for the decrease in pressure that was 171 00:10:15,300 --> 00:10:20,260 a result of this applied force to the system. 172 00:10:20,260 --> 00:10:28,470 And we could do this again in terms of q and k. 173 00:10:28,470 --> 00:10:33,150 Now we're going to get a little trickier. 174 00:10:33,150 --> 00:10:38,300 What happens if we add an inert gas to a system 175 00:10:38,300 --> 00:10:44,090 increasing the total pressure at constant temperature? 176 00:10:44,090 --> 00:10:46,860 So we're not adding one of the reactants or one of the 177 00:10:46,860 --> 00:10:49,420 products, we're adding an inert gas, and we're 178 00:10:49,420 --> 00:10:51,400 increasing the total pressure. 179 00:10:51,400 --> 00:10:58,290 What's going to happen? 180 00:10:58,290 --> 00:11:05,330 There's a couple of options, what do you think? 181 00:11:05,330 --> 00:11:09,200 How about nothing. 182 00:11:09,200 --> 00:11:13,330 Why would nothing happen? 183 00:11:13,330 --> 00:11:17,760 Well, q depends on the partial pressure of the reactant gas 184 00:11:17,760 --> 00:11:19,810 and the product gas. 185 00:11:19,810 --> 00:11:23,720 And in this particular example, the partial pressures 186 00:11:23,720 --> 00:11:25,470 are not changing. 187 00:11:25,470 --> 00:11:27,960 We are changing the total pressure of the system by 188 00:11:27,960 --> 00:11:30,220 adding an inert gas, but we're not 189 00:11:30,220 --> 00:11:33,870 changing the partial pressure. 190 00:11:33,870 --> 00:11:38,140 So let's take this opportunity to review partial pressures, 191 00:11:38,140 --> 00:11:40,440 or if you haven't seen it before, I'm going to tell you 192 00:11:40,440 --> 00:11:43,030 everything you need to know about partial pressures. 193 00:11:43,030 --> 00:11:48,580 So here's a little review, or for the first time, 194 00:11:48,580 --> 00:11:51,020 description of partial pressure. 195 00:11:51,020 --> 00:11:53,090 So what is the definition? 196 00:11:53,090 --> 00:11:57,000 The partial pressure is the pressure that each gas would 197 00:11:57,000 --> 00:12:03,060 exert if it were present alone in the container. 198 00:12:03,060 --> 00:12:07,010 So in this example, we have oxygen in a container, and we 199 00:12:07,010 --> 00:12:09,290 have one atmosphere of pressure. 200 00:12:09,290 --> 00:12:12,600 In the second one there's nitrogen in the container with 201 00:12:12,600 --> 00:12:15,470 one atomosphere of pressure. 202 00:12:15,470 --> 00:12:18,510 If you put this amount of oxygen and this amount of 203 00:12:18,510 --> 00:12:22,810 nitrogen together in a container, then you would have 204 00:12:22,810 --> 00:12:26,290 two atmospheres of total pressure, but you're still 205 00:12:26,290 --> 00:12:30,510 going to have one atmosphere of partial pressure for 206 00:12:30,510 --> 00:12:35,500 oxygen, and one atmosphere of partial pressure for nitrogen. 207 00:12:35,500 --> 00:12:39,220 So it's as if the gas is alone in the container. 208 00:12:39,220 --> 00:12:43,380 That's the definition of partial pressure. 209 00:12:43,380 --> 00:12:48,350 So let's look at some equations. 210 00:12:48,350 --> 00:12:52,760 So the partial pressure of a gas, p to sub a, is equal to 211 00:12:52,760 --> 00:12:57,360 the number of moles of that gas, and we also have in this 212 00:12:57,360 --> 00:13:00,320 equation, r, our gas constant, t, our 213 00:13:00,320 --> 00:13:02,980 temperature, and v, our volume. 214 00:13:02,980 --> 00:13:05,820 The total pressure on the system, which is what this two 215 00:13:05,820 --> 00:13:10,270 atmospheres is, is equal to the partial pressure of gas a, 216 00:13:10,270 --> 00:13:13,220 which would be the partial pressure of the oxygen at one 217 00:13:13,220 --> 00:13:15,990 atmosphere, the partial pressure of the nitrogen at 218 00:13:15,990 --> 00:13:18,360 one atmosphere, that's all we have here. 219 00:13:18,360 --> 00:13:21,920 So the partial pressure of those two, of one plus one, we 220 00:13:21,920 --> 00:13:24,950 have two for a total. 221 00:13:24,950 --> 00:13:29,910 And that's equal to the total number of the moles. 222 00:13:29,910 --> 00:13:31,940 So in these problems, we're going to be considering 223 00:13:31,940 --> 00:13:34,340 partial pressure, and the question you're going to be 224 00:13:34,340 --> 00:13:37,430 asking yourself in all of these different wordings of 225 00:13:37,430 --> 00:13:41,610 the question is is the partial pressure of the gas changing? 226 00:13:41,610 --> 00:13:43,940 That's a very important question to ask because 227 00:13:43,940 --> 00:13:50,260 that'll help you answer the questions correctly. 228 00:13:50,260 --> 00:13:53,780 So let's go back to the original question, what 229 00:13:53,780 --> 00:13:57,830 happens if an inert gas is added to the container, 230 00:13:57,830 --> 00:14:02,510 increasing the total pressure at constant temperature? 231 00:14:02,510 --> 00:14:07,240 And the answer is nothing, because q is not affected, 232 00:14:07,240 --> 00:14:10,770 because q depends on the partial pressures, and the 233 00:14:10,770 --> 00:14:12,670 partial pressure isn't changing. 234 00:14:12,670 --> 00:14:15,180 We aren't changing the number of moles of the gas in 235 00:14:15,180 --> 00:14:20,060 question, we're just changing the total pressure of the 236 00:14:20,060 --> 00:14:22,040 system, we're adding an inert gas. 237 00:14:22,040 --> 00:14:24,580 And because partial pressures don't change, q doesn't 238 00:14:24,580 --> 00:14:27,330 change, and if q doesn't change, there's no response 239 00:14:27,330 --> 00:14:30,580 from the system. 240 00:14:30,580 --> 00:14:34,290 So a lot of the questions in this unit, they're not 241 00:14:34,290 --> 00:14:37,680 actually very difficult, but the wording can be tricky. 242 00:14:37,680 --> 00:14:40,440 And so when you're doing these problems, you have to be 243 00:14:40,440 --> 00:14:44,210 thinking about what is changing and what isn't 244 00:14:44,210 --> 00:14:47,200 changing to be able to get these questions correct. 245 00:14:47,200 --> 00:14:50,690 And if you do that, then these are some nice questions for 246 00:14:50,690 --> 00:14:54,820 you to get right on the exams -- they're short, they're good 247 00:14:54,820 --> 00:14:59,430 questions to go after. 248 00:14:59,430 --> 00:15:00,770 All right. 249 00:15:00,770 --> 00:15:03,400 Let's see how you're doing with this. 250 00:15:03,400 --> 00:15:07,670 What happens if an inert gas is added to the container, but 251 00:15:07,670 --> 00:15:11,550 the total pressure is now kept constant, 252 00:15:11,550 --> 00:15:14,040 temperature is also constant. 253 00:15:14,040 --> 00:15:15,040 So let's think about this. 254 00:15:15,040 --> 00:15:57,480 OK, let's take 10 more seconds. 255 00:15:57,480 --> 00:16:01,990 OK, we're going down into the 60's -- again, we're heading 256 00:16:01,990 --> 00:16:04,270 toward the 90's by the end, but this is actually new 257 00:16:04,270 --> 00:16:07,890 material, so that's fine. 258 00:16:07,890 --> 00:16:09,900 All right, so let's think about it. 259 00:16:09,900 --> 00:16:13,580 So the reaction's going to shift toward reactants. 260 00:16:13,580 --> 00:16:15,490 Let's think about why, let's break it down. 261 00:16:15,490 --> 00:16:19,030 And what was important is that hint that for the pressure to 262 00:16:19,030 --> 00:16:21,470 be kept constant, the volume of the 263 00:16:21,470 --> 00:16:23,830 container must have increased. 264 00:16:23,830 --> 00:16:27,000 So let's take a look at that. 265 00:16:27,000 --> 00:16:31,960 So if the total pressure was kept constant, if the total 266 00:16:31,960 --> 00:16:35,500 pressure didn't change when you added an inert gas, the 267 00:16:35,500 --> 00:16:37,020 volume must have changed. 268 00:16:37,020 --> 00:16:38,060 Let's look at this. 269 00:16:38,060 --> 00:16:41,580 Say oxygen is our gas of interest, and n 270 00:16:41,580 --> 00:16:43,540 2 is an inert gas. 271 00:16:43,540 --> 00:16:47,470 If we added an inert gas to our system, the total pressure 272 00:16:47,470 --> 00:16:48,880 should go up. 273 00:16:48,880 --> 00:16:52,680 If it doesn't, then something else must have changed, and 274 00:16:52,680 --> 00:16:55,590 what must have changed is the volume. 275 00:16:55,590 --> 00:16:58,390 Otherwise the total pressure would not 276 00:16:58,390 --> 00:17:01,250 have stayed the same. 277 00:17:01,250 --> 00:17:05,360 So the volume of the container must have increased if the 278 00:17:05,360 --> 00:17:09,630 pressure is the same. 279 00:17:09,630 --> 00:17:16,220 So then we asked the question, if you increase the volume, 280 00:17:16,220 --> 00:17:19,970 what happens to the reaction, how does it shift? 281 00:17:19,970 --> 00:17:25,670 If you increase the volume, if you expand the volume, then 282 00:17:25,670 --> 00:17:29,760 you're going to shift from, in this case where you have one 283 00:17:29,760 --> 00:17:34,610 mole to two moles toward reactants, you will, as the 284 00:17:34,610 --> 00:17:38,360 volume increases, then you're going to have a change in the 285 00:17:38,360 --> 00:17:41,440 partial pressure of the gas -- all of a sudden the gas has a 286 00:17:41,440 --> 00:17:46,150 lot more room, and its pressure is going to decrease, 287 00:17:46,150 --> 00:17:48,320 the partial pressure will decrease. 288 00:17:48,320 --> 00:17:51,970 It has a lot more space for itself -- it's like it's in 289 00:17:51,970 --> 00:17:53,420 there all by itself. 290 00:17:53,420 --> 00:17:55,790 Partial pressure, you can think of it as selfish gas 291 00:17:55,790 --> 00:17:57,690 molecules, they don't care what else is there. 292 00:17:57,690 --> 00:18:00,730 They're only thinking about how they're fairing in this 293 00:18:00,730 --> 00:18:02,370 environment. 294 00:18:02,370 --> 00:18:06,010 So then we want to shift in a way that compensates for that 295 00:18:06,010 --> 00:18:08,850 stress that increases the pressure, so we move from one 296 00:18:08,850 --> 00:18:12,260 molecule to two molecules. 297 00:18:12,260 --> 00:18:17,350 So it's all about the partial pressure q and k. 298 00:18:17,350 --> 00:18:19,920 So again in these questions you have to dissect 299 00:18:19,920 --> 00:18:20,990 what's going on. 300 00:18:20,990 --> 00:18:22,460 What has changed? 301 00:18:22,460 --> 00:18:24,790 The questions are often phrased in such a way, 302 00:18:24,790 --> 00:18:27,680 something's kept constant, but if something's kept constant, 303 00:18:27,680 --> 00:18:30,670 you have to ask the question, to keep it constant, did 304 00:18:30,670 --> 00:18:33,360 something have to change for that to be true. 305 00:18:33,360 --> 00:18:35,500 So again, the wordings of these can be tricky. 306 00:18:35,500 --> 00:18:36,020 Do you have a question? 307 00:18:36,020 --> 00:18:39,920 STUDENT: [INAUDIBLE] 308 00:18:39,920 --> 00:18:47,630 PROFESSOR: So, the question is is it necessary for it to try 309 00:18:47,630 --> 00:18:48,680 to increase pressure. 310 00:18:48,680 --> 00:18:50,490 This is just the predictive tool of Le Chatelier. 311 00:18:50,490 --> 00:18:54,420 Le Chatelier would predict that if the system has changed 312 00:18:54,420 --> 00:18:56,870 in such a way, such as volume increases so the partial 313 00:18:56,870 --> 00:19:00,020 pressure decreases, that the system will shift in a way to 314 00:19:00,020 --> 00:19:01,560 minimize that stress. 315 00:19:01,560 --> 00:19:05,180 And to minimize that stress, you would increase the partial 316 00:19:05,180 --> 00:19:07,910 pressure by doing the shift from one mole to two. 317 00:19:07,910 --> 00:19:12,120 So again, this is Le Chatelier's predictive, how 318 00:19:12,120 --> 00:19:14,600 you would predict the direction of the shift based 319 00:19:14,600 --> 00:19:17,810 on the simple idea that a system where a stress is 320 00:19:17,810 --> 00:19:21,350 applied, the system will respond in a way to minimize 321 00:19:21,350 --> 00:19:22,670 the stress. 322 00:19:22,670 --> 00:19:26,570 And again, I talked about last time that this concept of 323 00:19:26,570 --> 00:19:30,610 minimizing stress is difficult for some MIT students, but you 324 00:19:30,610 --> 00:19:33,490 just sort of have to memorize that in this case we're 325 00:19:33,490 --> 00:19:43,390 predicting how a system will shift to minimize that stress. 326 00:19:43,390 --> 00:19:45,390 OK. 327 00:19:45,390 --> 00:19:46,880 So if you need a review of partial 328 00:19:46,880 --> 00:19:48,410 pressure, go over this. 329 00:19:48,410 --> 00:19:51,910 This part, it looks a little challenging at first, and some 330 00:19:51,910 --> 00:19:54,250 of the questions you really have to sit down and dissect 331 00:19:54,250 --> 00:19:57,830 what's going on, and for most people, once they get there, 332 00:19:57,830 --> 00:20:00,590 that part, those are good questions, you look forward to 333 00:20:00,590 --> 00:20:04,000 them on the exam. 334 00:20:04,000 --> 00:20:07,970 So now, we've talked about adding reagents, we've talked 335 00:20:07,970 --> 00:20:11,360 about removing reagents, removing products, adding 336 00:20:11,360 --> 00:20:16,780 products, shifts in volume, and now we're going to talk 337 00:20:16,780 --> 00:20:20,370 about changing the temperature. 338 00:20:20,370 --> 00:20:23,510 So here, Le Chatelier's principle, it's a little bit 339 00:20:23,510 --> 00:20:28,460 more fuzzy, but it still basically works. 340 00:20:28,460 --> 00:20:32,950 So if you talk about raising the temperature of a mixture 341 00:20:32,950 --> 00:20:37,720 at equilibrium, then Le Chatelier's principle would 342 00:20:37,720 --> 00:20:41,360 suggest that the system is going to respond in such a way 343 00:20:41,360 --> 00:20:42,940 to minimize that stress. 344 00:20:42,940 --> 00:20:47,046 So if you're adding heat, the system wants to respond in a 345 00:20:47,046 --> 00:20:51,050 way to absorb some of the heat, to counterbalance that 346 00:20:51,050 --> 00:20:54,500 change to the system. 347 00:20:54,500 --> 00:20:57,530 So let's take a look at this. 348 00:20:57,530 --> 00:21:01,660 So let's think about raising the temperature of an 349 00:21:01,660 --> 00:21:07,270 exothermic reaction. 350 00:21:07,270 --> 00:21:09,710 So what's going to happen here? 351 00:21:09,710 --> 00:21:14,100 Raising the temperature of an exothermic reaction, favors 352 00:21:14,100 --> 00:21:18,470 the formation of reactants, would shift toward reactants. 353 00:21:18,470 --> 00:21:20,450 Well, why would this be true. 354 00:21:20,450 --> 00:21:24,900 Well, you can think about this very simplistically that in an 355 00:21:24,900 --> 00:21:29,920 exothermic reaction, you're going to be producing heat in 356 00:21:29,920 --> 00:21:33,730 the forward direction and absorbing heat 357 00:21:33,730 --> 00:21:35,640 in the reverse direction. 358 00:21:35,640 --> 00:21:38,670 So if it's an exothermic reaction, that means it's 359 00:21:38,670 --> 00:21:41,440 exothermic in the forward direction, which would mean it 360 00:21:41,440 --> 00:21:45,290 was endothermic in the reverse direction. 361 00:21:45,290 --> 00:21:48,390 So if you raise the temperature of an exothermic 362 00:21:48,390 --> 00:21:52,570 reaction, you're adding heat, the system wants to respond in 363 00:21:52,570 --> 00:21:58,760 such a way to absorb that heat, so it would shift toward 364 00:21:58,760 --> 00:22:01,220 the endothermic direction of the reaction, which is the 365 00:22:01,220 --> 00:22:05,300 reverse direction toward reactants. 366 00:22:05,300 --> 00:22:09,290 So add heat, system wants to shift to absorb 367 00:22:09,290 --> 00:22:10,790 some of that heat. 368 00:22:10,790 --> 00:22:13,450 So in this part you think about the direction of the 369 00:22:13,450 --> 00:22:16,350 reaction, whether it's exothermic or endothermic, and 370 00:22:16,350 --> 00:22:19,230 then think about compensating for that stress. 371 00:22:19,230 --> 00:22:25,860 Endothermic reaction, again, they say it's an endothermic 372 00:22:25,860 --> 00:22:27,130 reaction, it means it's endothermic 373 00:22:27,130 --> 00:22:29,270 in the forward direction. 374 00:22:29,270 --> 00:22:32,170 So heat is being absorbed in the forward direction -- again 375 00:22:32,170 --> 00:22:35,330 this is just sort of a very simplistic way 376 00:22:35,330 --> 00:22:37,240 to think about it. 377 00:22:37,240 --> 00:22:40,330 So if you raise the temperature then, you want to 378 00:22:40,330 --> 00:22:44,760 absorb that heat, according to Le Chatelier's principle, and 379 00:22:44,760 --> 00:22:51,760 so that would favor a shift toward products. 380 00:22:51,760 --> 00:22:56,560 So let's look some more at this. 381 00:22:56,560 --> 00:23:01,570 The predictive tool here is delta h. 382 00:23:01,570 --> 00:23:04,200 So delta h, whether the reaction is exothermic or 383 00:23:04,200 --> 00:23:08,630 endothermic, is going to be a predictive tool in thinking 384 00:23:08,630 --> 00:23:13,930 about the direction of change, when heat is added to a system 385 00:23:13,930 --> 00:23:21,490 or heat is removed from a system. 386 00:23:21,490 --> 00:23:24,710 So let's try this out. 387 00:23:24,710 --> 00:23:27,030 Heat is added to a system. 388 00:23:27,030 --> 00:23:30,680 You're given information about your predictive tool, delta h. 389 00:23:30,680 --> 00:23:49,340 Which direction will the reaction go? 390 00:23:49,340 --> 00:24:06,340 All right, 10 seconds. 391 00:24:06,340 --> 00:24:42,300 Why don't you discuss with your friends and vote again. 392 00:24:42,300 --> 00:24:55,930 All right, 10 more seconds. 393 00:24:55,930 --> 00:24:58,480 Yes! 394 00:24:58,480 --> 00:25:02,940 I knew we could get into the 90's. 395 00:25:02,940 --> 00:25:06,280 All right, so what type of reaction is this, endo or 396 00:25:06,280 --> 00:25:07,140 exothermic? 397 00:25:07,140 --> 00:25:08,040 Exothermic. 398 00:25:08,040 --> 00:25:13,570 And so we're adding heat to an exothermic reaction, so it 399 00:25:13,570 --> 00:25:17,350 wants to shift to absorb that heat, so it wants to go in 400 00:25:17,350 --> 00:25:20,950 endothermic direction, which is the reverse direction or 401 00:25:20,950 --> 00:25:24,040 toward the reactants. 402 00:25:24,040 --> 00:25:26,380 And that's how it's supposed to work too, more people get 403 00:25:26,380 --> 00:25:30,950 the right answer after the group discusses. 404 00:25:30,950 --> 00:25:34,530 All right. 405 00:25:34,530 --> 00:25:37,700 So we've been talking about the equilibrium constant so 406 00:25:37,700 --> 00:25:42,740 far in terms of it being a constant, it's called the 407 00:25:42,740 --> 00:25:46,850 equilibrium constant, but it's only constant at a given 408 00:25:46,850 --> 00:25:47,830 temperature. 409 00:25:47,830 --> 00:25:53,670 So the equilibrium constant changes with temperature. 410 00:25:53,670 --> 00:25:57,110 It's also true that rates of reaction change with 411 00:25:57,110 --> 00:25:58,090 temperature. 412 00:25:58,090 --> 00:26:01,440 And kinetics is our last unit in this course, and we'll be 413 00:26:01,440 --> 00:26:04,470 discussing that quite a bit. 414 00:26:04,470 --> 00:26:08,870 So how does the equilibrium constant change with 415 00:26:08,870 --> 00:26:09,980 temperature? 416 00:26:09,980 --> 00:26:12,050 Let's consider that. 417 00:26:12,050 --> 00:26:15,570 So let's look at some of the equations that we know that 418 00:26:15,570 --> 00:26:19,860 have delta h in them -- delta h, again, is our predictive 419 00:26:19,860 --> 00:26:23,470 factor when we're talking about changes in temperature. 420 00:26:23,470 --> 00:26:28,180 We know that delta g nought is minus r t natural log of k, so 421 00:26:28,180 --> 00:26:31,720 the relationship with our standard free energy, our gas 422 00:26:31,720 --> 00:26:35,300 constant temperature, and our equilibrium constant. 423 00:26:35,300 --> 00:26:38,640 We also know that delta g nought equals delta h nought 424 00:26:38,640 --> 00:26:40,630 minus t delta s nought. 425 00:26:40,630 --> 00:26:44,340 If those equations don't look familiar, then you should go 426 00:26:44,340 --> 00:26:48,450 back and review the material on thermodynamics because 427 00:26:48,450 --> 00:26:51,300 you're going to need it in this unit, the next unit, the 428 00:26:51,300 --> 00:26:54,500 unit after that, and then the next two units, and they we're 429 00:26:54,500 --> 00:26:55,600 done with the class. 430 00:26:55,600 --> 00:26:59,450 All right, so good to get familiar with this now. 431 00:26:59,450 --> 00:27:04,695 All right, so we can rearrange these in terms of solving for 432 00:27:04,695 --> 00:27:07,090 our equilibrium constant. 433 00:27:07,090 --> 00:27:10,220 And so we see the natural log of the equilibrium constant 434 00:27:10,220 --> 00:27:15,980 minus delta h nought over r t, plus delta s nought over r. 435 00:27:15,980 --> 00:27:19,970 And so, it's reasonable to assume that for the things 436 00:27:19,970 --> 00:27:23,640 we're talking about, that delta h nought and delta s 437 00:27:23,640 --> 00:27:26,690 nought are pretty much independent of temperatures. 438 00:27:26,690 --> 00:27:29,270 That would be true for pretty much any temperature we would 439 00:27:29,270 --> 00:27:30,870 be talking about. 440 00:27:30,870 --> 00:27:36,340 So that means that k, or the natural log of k, is going to 441 00:27:36,340 --> 00:27:38,420 change depending on the temperature. 442 00:27:38,420 --> 00:27:41,290 It won't change -- these other ones are constant at all the 443 00:27:41,290 --> 00:27:44,300 temperatures, so there's a relationship between 444 00:27:44,300 --> 00:27:50,770 equilibrium constant and temperature. 445 00:27:50,770 --> 00:27:54,190 So you can consider an equilibrium constant at one 446 00:27:54,190 --> 00:27:57,100 temperature, and an equilibrium constant at 447 00:27:57,100 --> 00:27:58,910 another temperature. 448 00:27:58,910 --> 00:28:03,900 So we can talk about the natural log of a k 2, of an 449 00:28:03,900 --> 00:28:08,670 equilibrium constant 2, equal to minus delta h over r t 450 00:28:08,670 --> 00:28:12,980 temperature 2, plus delta s nought over r. 451 00:28:12,980 --> 00:28:17,230 We can do the same thing for another temperature, another 452 00:28:17,230 --> 00:28:23,050 temperature 1, equilibrium 1, and if we subtract these two, 453 00:28:23,050 --> 00:28:26,680 delta s is going to cancel out, and we're going to get 454 00:28:26,680 --> 00:28:30,350 this equation, which you know is important because it has a 455 00:28:30,350 --> 00:28:34,530 name, so this is the van't Hoff equation. 456 00:28:34,530 --> 00:28:37,380 And at some point later in the course, I'll be asking you to 457 00:28:37,380 --> 00:28:40,790 come up with this name, so I always get very excited when 458 00:28:40,790 --> 00:28:43,740 people can tell me it's a van't Hoff equation later on, 459 00:28:43,740 --> 00:28:46,740 not that that's on a test, but there are very few equations 460 00:28:46,740 --> 00:28:48,890 that have names. 461 00:28:48,890 --> 00:28:50,260 So here we go. 462 00:28:50,260 --> 00:28:51,180 So what does this do? 463 00:28:51,180 --> 00:28:54,960 Well, we can talk about the natural log of one equilibrium 464 00:28:54,960 --> 00:28:58,020 constant over another, an equilibrium constant k 2, 465 00:28:58,020 --> 00:29:01,080 which is the equilibrium at temperature 2, equilibrium 466 00:29:01,080 --> 00:29:03,760 constant k 1, which is the equilibrium constant at 467 00:29:03,760 --> 00:29:08,100 temperature 1, is going to be equal to minus delta h nought 468 00:29:08,100 --> 00:29:11,270 over r, and then it depends on the temperature. 469 00:29:11,270 --> 00:29:14,680 So you bracket 1 over temperature 2 minus 1 over 470 00:29:14,680 --> 00:29:15,740 temperature 1. 471 00:29:15,740 --> 00:29:19,950 And this just comes from subtracting those 2 equations. 472 00:29:19,950 --> 00:29:23,920 So again, you can think about how the equilibrium constant's 473 00:29:23,920 --> 00:29:26,990 going to change with temperature -- that's what 474 00:29:26,990 --> 00:29:30,090 this equation does for you. 475 00:29:30,090 --> 00:29:34,640 So let's look at some of the things that fall out of this 476 00:29:34,640 --> 00:29:39,160 particular equation. 477 00:29:39,160 --> 00:29:44,140 Let's think first about a case where this delta h nought is 478 00:29:44,140 --> 00:29:45,710 less than zero. 479 00:29:45,710 --> 00:29:49,830 What type of reaction is that? 480 00:29:49,830 --> 00:29:50,530 Exothermic. 481 00:29:50,530 --> 00:29:54,480 All right, so now let's consider a case for this 482 00:29:54,480 --> 00:29:59,760 exothermic reaction where we're going to increase the 483 00:29:59,760 --> 00:30:03,840 temperature, and so that means that t 2 will be greater than 484 00:30:03,840 --> 00:30:08,070 t 1 -- we've increased the temperature. 485 00:30:08,070 --> 00:30:12,850 So now let's think about what sign the natural log of k 2 486 00:30:12,850 --> 00:30:19,060 over k 1 will have if these things are true. 487 00:30:19,060 --> 00:30:21,820 So there's a minus in the equation, let's put 488 00:30:21,820 --> 00:30:23,640 the minus down here. 489 00:30:23,640 --> 00:30:27,225 Delta h nought is negative, it's exothermic reaction, so 490 00:30:27,225 --> 00:30:29,960 that's another negative sign. 491 00:30:29,960 --> 00:30:33,570 And if we're increasing the temperature, and t 2 is 492 00:30:33,570 --> 00:30:36,980 greater than t 1, then this temperature term will also 493 00:30:36,980 --> 00:30:39,060 have and negative sign. 494 00:30:39,060 --> 00:30:42,580 So if you have negative times negative times negative, 495 00:30:42,580 --> 00:30:47,460 you're going to get a negative for your net result over here. 496 00:30:47,460 --> 00:30:52,530 And what that will mean in terms of k 2 and k 1, is that 497 00:30:52,530 --> 00:30:57,260 k 1 will be greater than k 2. 498 00:30:57,260 --> 00:31:00,610 So you can think about this mathematically by running 499 00:31:00,610 --> 00:31:02,200 through this argument. 500 00:31:02,200 --> 00:31:06,590 You can also sort of think about it in terms of what you 501 00:31:06,590 --> 00:31:12,000 would expect if you increase the temperature in terms of 502 00:31:12,000 --> 00:31:16,140 what's the relative ratio of products to reactants at 503 00:31:16,140 --> 00:31:20,760 equilibrium for an exothermic reaction. 504 00:31:20,760 --> 00:31:24,800 So if you're increasing the temperature of an exothermic 505 00:31:24,800 --> 00:31:29,240 reaction, the reaction would want to shift in the direction 506 00:31:29,240 --> 00:31:33,870 to absorb that heat in the endothermic direction. 507 00:31:33,870 --> 00:31:38,690 So you would expect that the equilibrium constant that was 508 00:31:38,690 --> 00:31:41,170 at the lower temperature would be larger than the higher 509 00:31:41,170 --> 00:31:45,130 temperature one, that there'd be less products at this new 510 00:31:45,130 --> 00:31:47,260 higher temperature. 511 00:31:47,260 --> 00:31:50,480 So again, you can think about it in terms of Le Chatelier's 512 00:31:50,480 --> 00:31:53,600 principle, or you can run through with this equation and 513 00:31:53,600 --> 00:31:56,540 do the math and think about the relative size 514 00:31:56,540 --> 00:32:02,140 of k 1 and k 2. 515 00:32:02,140 --> 00:32:04,970 So now let's think about what happens when you decrease the 516 00:32:04,970 --> 00:32:09,530 temperature where t 2 is less than t 1. 517 00:32:09,530 --> 00:32:14,040 So we can do the same thing, we have a negative sign here, 518 00:32:14,040 --> 00:32:16,510 we also have a negative sign for delta h, it's an 519 00:32:16,510 --> 00:32:18,030 exothermic reaction. 520 00:32:18,030 --> 00:32:20,560 But now we have a positive sign for 521 00:32:20,560 --> 00:32:22,100 this temperature term. 522 00:32:22,100 --> 00:32:25,500 So overall we're going to have a positive sign for the 523 00:32:25,500 --> 00:32:29,950 natural log of k 2 over k 1, which mathematically is going 524 00:32:29,950 --> 00:32:35,180 to mean that k 1 is less than k 2 or that you would expect 525 00:32:35,180 --> 00:32:40,360 more products in the equilibrium constant at higher 526 00:32:40,360 --> 00:32:41,110 temperature. 527 00:32:41,110 --> 00:32:49,190 All right, so that's an exothermic reaction. 528 00:32:49,190 --> 00:32:51,960 So if you're ever having problems with Le Chatelier, 529 00:32:51,960 --> 00:32:55,430 you can go and use the van't Hoff equation to think about 530 00:32:55,430 --> 00:32:58,910 what you would expect in terms of this shift, what you would 531 00:32:58,910 --> 00:33:01,770 expect if you change the temperature of an exothermic 532 00:33:01,770 --> 00:33:05,020 reaction in terms of the magnitude of the new 533 00:33:05,020 --> 00:33:06,410 equilibrium constants. 534 00:33:06,410 --> 00:33:15,280 All right, now you can do the same for me in terms of this 535 00:33:15,280 --> 00:33:18,700 delta h nought is greater than zero. 536 00:33:18,700 --> 00:33:22,780 Which of the following are going to be true, and notice 537 00:33:22,780 --> 00:33:27,190 there's a possibility that all of them are true or that just 538 00:33:27,190 --> 00:33:30,520 some of them are true or that one of them is true. 539 00:33:30,520 --> 00:34:46,690 All right, take 10 more seconds. 540 00:34:46,690 --> 00:34:49,480 See, actually people did very well, because e is also true, 541 00:34:49,480 --> 00:34:53,100 all of those are true, but d was also true. 542 00:34:53,100 --> 00:34:58,840 So if we add the 30 and the 58 together, I'm very happy. 543 00:34:58,840 --> 00:35:03,580 All right, so let's look at this -- we'll keep the 544 00:35:03,580 --> 00:35:06,790 questions up and let's consider all of them in order. 545 00:35:06,790 --> 00:35:13,080 So the first one is that the reaction was endothermic, I 546 00:35:13,080 --> 00:35:16,580 think everybody saw that as being correct. 547 00:35:16,580 --> 00:35:19,540 OK, the second one talked about the equilibrium constant 548 00:35:19,540 --> 00:35:21,570 is higher at higher temperatures. 549 00:35:21,570 --> 00:35:22,900 So let's look at the first one. 550 00:35:22,900 --> 00:35:27,750 We increased the temperature here, and we see that if you 551 00:35:27,750 --> 00:35:31,240 work out the math or just think about it, k 2 is larger 552 00:35:31,240 --> 00:35:35,020 than k 1, so the second equilibrium constant is 553 00:35:35,020 --> 00:35:39,000 greater than the first when you increase the temperature. 554 00:35:39,000 --> 00:35:42,440 So that would favor then, in an endothermic direction, if 555 00:35:42,440 --> 00:35:44,810 you're increasing the temperature, it favors the 556 00:35:44,810 --> 00:35:47,760 endothermic direction of the reaction, so you could think 557 00:35:47,760 --> 00:35:52,100 about the ratio having more products at that new 558 00:35:52,100 --> 00:35:55,410 equilibrium constant at that higher temperature. 559 00:35:55,410 --> 00:35:57,520 So that one was also true. 560 00:35:57,520 --> 00:36:00,060 Then let's think about when you decrease the temperature, 561 00:36:00,060 --> 00:36:05,140 when temperature 1 is greater than temperature 2. 562 00:36:05,140 --> 00:36:09,660 And so this would favor, then, the exothermic direction. 563 00:36:09,660 --> 00:36:14,860 So you would expect less products at this newer 564 00:36:14,860 --> 00:36:16,100 temperature. 565 00:36:16,100 --> 00:36:21,090 So, this part asked about k 1 versus k 2, and we see k 1 is 566 00:36:21,090 --> 00:36:22,870 greater than k 2. 567 00:36:22,870 --> 00:36:26,340 And then d says there's fewer products and equilibrium when 568 00:36:26,340 --> 00:36:30,630 the temperature is decreased, and that's just the word way 569 00:36:30,630 --> 00:36:33,080 of saying the same thing. 570 00:36:33,080 --> 00:36:36,520 So all of those are true. 571 00:36:36,520 --> 00:36:42,440 But again, people did very, very well with this question. 572 00:36:42,440 --> 00:36:45,190 All right. 573 00:36:45,190 --> 00:36:50,280 So now we're going to combine what we learned on Friday with 574 00:36:50,280 --> 00:36:55,855 what we learned today, and think about ways that that can 575 00:36:55,855 --> 00:36:57,170 be applied. 576 00:36:57,170 --> 00:37:01,080 So why should you care about Le Chatelier's principle? 577 00:37:01,080 --> 00:37:04,590 Well, for two reasons -- one I think Le Chatelier had sort of 578 00:37:04,590 --> 00:37:08,030 a good life plan that when stress is applied to the 579 00:37:08,030 --> 00:37:12,300 system, one should respond in a way to minimize that stress. 580 00:37:12,300 --> 00:37:15,470 I think that is an important life lesson. 581 00:37:15,470 --> 00:37:19,540 But also, it's very useful in terms of thinking about 582 00:37:19,540 --> 00:37:22,010 maximizing a yield of a reaction. 583 00:37:22,010 --> 00:37:26,090 So if you were going to create an industrial process and make 584 00:37:26,090 --> 00:37:31,530 lots of money and give some of it back to MIT to improve 585 00:37:31,530 --> 00:37:36,250 things in terms of teaching chemistry at MIT, then you 586 00:37:36,250 --> 00:37:38,850 would want to think about these principles, because you 587 00:37:38,850 --> 00:37:40,890 would want to maximize your yield. 588 00:37:40,890 --> 00:37:42,610 If you're going to be making a product, you want to make a 589 00:37:42,610 --> 00:37:45,790 lot of that product, and so you want to be thinking about 590 00:37:45,790 --> 00:37:47,130 these things. 591 00:37:47,130 --> 00:37:51,870 So we've talked about the reaction with nitrogen and 592 00:37:51,870 --> 00:37:54,720 hydrogen making ammonia last time. 593 00:37:54,720 --> 00:37:58,930 And this is an exothermic reaction. 594 00:37:58,930 --> 00:38:03,190 So, there are a lot of people who want to make ammonia -- 595 00:38:03,190 --> 00:38:06,490 it's for fertilizer, and as some of you have heard in 596 00:38:06,490 --> 00:38:10,770 terms of terrorism, it also can be used as an explosive, 597 00:38:10,770 --> 00:38:13,890 so there are a lot of people who want to 598 00:38:13,890 --> 00:38:16,400 make a lot of this. 599 00:38:16,400 --> 00:38:19,860 So how do you maximize this yield? 600 00:38:19,860 --> 00:38:22,380 It's an exothermic reaction. 601 00:38:22,380 --> 00:38:25,910 So you can think about temperature. 602 00:38:25,910 --> 00:38:32,680 So here, low temperature would favor product, which is good. 603 00:38:32,680 --> 00:38:35,820 We haven't talked about kinetics yet, but low 604 00:38:35,820 --> 00:38:39,090 temperature also slows the rate, which is bad. 605 00:38:39,090 --> 00:38:41,600 Because you not only want to make a lot of your product, 606 00:38:41,600 --> 00:38:44,930 you want to make it in a reasonable time frame and sell 607 00:38:44,930 --> 00:38:47,300 it and make lots of money and retire early. 608 00:38:47,300 --> 00:38:51,460 So you care about how fast the reaction is going as well. 609 00:38:51,460 --> 00:38:55,670 So for an exothermic reaction then, you have to balance out 610 00:38:55,670 --> 00:38:58,700 what's thermodynamically favorable and what's 611 00:38:58,700 --> 00:39:00,500 kinetically favorable. 612 00:39:00,500 --> 00:39:04,530 So thermodynamically, you would want a low temperature, 613 00:39:04,530 --> 00:39:08,640 but kinetically in terms of the rate, that's not so good. 614 00:39:08,640 --> 00:39:12,570 So they use a compromised temperature, which is 500 615 00:39:12,570 --> 00:39:14,410 degrees c, so that's pretty high 616 00:39:14,410 --> 00:39:18,430 actually for this reaction. 617 00:39:18,430 --> 00:39:21,910 So what are other things that you could do to drive this 618 00:39:21,910 --> 00:39:26,070 reaction toward products? 619 00:39:26,070 --> 00:39:27,990 So yeah, what's something else you could do? 620 00:39:27,990 --> 00:39:31,200 STUDENT: Increase the volume. 621 00:39:31,200 --> 00:39:31,540 PROFESSOR: Right. 622 00:39:31,540 --> 00:39:33,680 So you could think about volume here. 623 00:39:33,680 --> 00:39:39,350 So here we have four molecules of gas on one side, and two 624 00:39:39,350 --> 00:39:42,140 molecules of gas on the other side. 625 00:39:42,140 --> 00:39:44,800 So you can think about changing that 626 00:39:44,800 --> 00:39:48,180 to favor your products. 627 00:39:48,180 --> 00:39:49,550 What is something else you could do? 628 00:39:49,550 --> 00:39:51,670 STUDENT: Enzymes. 629 00:39:51,670 --> 00:39:56,180 PROFESSOR: Enzymes, we'll talk about that in a minute. 630 00:39:56,180 --> 00:39:58,570 If you can't use the enzymes, what's something else 631 00:39:58,570 --> 00:39:59,290 that you could do? 632 00:39:59,290 --> 00:40:02,340 STUDENT: You could remove ammonia from the system. 633 00:40:02,340 --> 00:40:04,060 PROFESSOR: Yup, so you could remove ammonia from the 634 00:40:04,060 --> 00:40:07,260 system, which would also shift the direction. 635 00:40:07,260 --> 00:40:10,110 So let's just put those two things down. 636 00:40:10,110 --> 00:40:11,750 Remove products. 637 00:40:11,750 --> 00:40:14,410 So if you remove ammonia from the system, and they actually 638 00:40:14,410 --> 00:40:17,660 do this in the industrial process, so they'll stop -- 639 00:40:17,660 --> 00:40:21,200 they'll, at a certain time points, they will remove the 640 00:40:21,200 --> 00:40:23,070 product and they'll say hey, let's shift the 641 00:40:23,070 --> 00:40:24,840 reaction to make more. 642 00:40:24,840 --> 00:40:28,870 You just changed q versus k, you removed your product, so 643 00:40:28,870 --> 00:40:32,650 now we want to shift to minimize that stress and make 644 00:40:32,650 --> 00:40:33,870 more product. 645 00:40:33,870 --> 00:40:37,980 And the first response, compress the volume, so we go 646 00:40:37,980 --> 00:40:41,610 from four molecules to two molecules, and that would be 647 00:40:41,610 --> 00:40:44,720 favor -- again, the system would want to respond, you 648 00:40:44,720 --> 00:40:49,560 compressed the volume, you increased the pressure, so you 649 00:40:49,560 --> 00:40:53,460 want to respond in such a way that compensates for that 650 00:40:53,460 --> 00:40:56,700 increased pressure, which is decrease the pressure by going 651 00:40:56,700 --> 00:40:59,160 from the four molecules to two. 652 00:40:59,160 --> 00:41:02,310 So both of those are great, and both of those are, in 653 00:41:02,310 --> 00:41:03,470 fact, used. 654 00:41:03,470 --> 00:41:06,790 Sometimes when you study chemistry you'll realize that 655 00:41:06,790 --> 00:41:11,180 some of the -- people made a lot of money on this, and that 656 00:41:11,180 --> 00:41:13,720 some of the principles are actually very simple. 657 00:41:13,720 --> 00:41:16,660 You are learning principles in this class that, applied 658 00:41:16,660 --> 00:41:19,420 correctly, could make you a lot of money. 659 00:41:19,420 --> 00:41:21,960 Some of the scientific discoveries are actually not 660 00:41:21,960 --> 00:41:23,350 all that complicated. 661 00:41:23,350 --> 00:41:26,980 All right. 662 00:41:26,980 --> 00:41:29,310 So someone said use enzymes. 663 00:41:29,310 --> 00:41:32,180 I love that answer, I want to use enzymes to do a lot of 664 00:41:32,180 --> 00:41:35,630 those things, and people are trying to do this. 665 00:41:35,630 --> 00:41:39,070 Now, why would you want to use an enzyme? 666 00:41:39,070 --> 00:41:42,860 Well, there is a lot of nitrogen in the air, but 667 00:41:42,860 --> 00:41:47,040 there's not a lot of usable nitrogen, because n 2 gas is 668 00:41:47,040 --> 00:41:50,400 in the air, but it's very difficult to break it apart to 669 00:41:50,400 --> 00:41:52,530 form ammonia. 670 00:41:52,530 --> 00:41:55,680 And when you think about bonding, you can probably tell 671 00:41:55,680 --> 00:42:00,370 me why it's hard to break n 2 apart, and if you consider how 672 00:42:00,370 --> 00:42:04,870 many bonds would be between those two nitrogens. 673 00:42:04,870 --> 00:42:08,070 So it's there but it's hard to do. 674 00:42:08,070 --> 00:42:11,470 So the industrial process that's used now pretty much is 675 00:42:11,470 --> 00:42:14,465 the same industrial process that's been used for quite a 676 00:42:14,465 --> 00:42:16,820 while, the Haber-Bosch process, which 677 00:42:16,820 --> 00:42:18,970 does not use enzymes. 678 00:42:18,970 --> 00:42:22,990 And here are some pictures of these folks. 679 00:42:22,990 --> 00:42:28,380 This resulted in two Noble prizes, the development of the 680 00:42:28,380 --> 00:42:32,200 industrial process here, and it's still being used. 681 00:42:32,200 --> 00:42:37,250 On a historic note, these were German scientists, and so, 682 00:42:37,250 --> 00:42:40,660 working out this process was important to the Germans 683 00:42:40,660 --> 00:42:42,420 during World War II. 684 00:42:42,420 --> 00:42:47,640 It's still the process going on today. 685 00:42:47,640 --> 00:42:49,460 So what about enzymes. 686 00:42:49,460 --> 00:42:52,770 Maybe enzymes could do a bit better than this process. 687 00:42:52,770 --> 00:42:54,530 It's not true yet. 688 00:42:54,530 --> 00:42:58,300 Still the Haber-Bosch industrial process is the one 689 00:42:58,300 --> 00:43:02,200 that's being used, but enzymes can do it. 690 00:43:02,200 --> 00:43:07,630 So, some of the problems of a lot of the common industrial 691 00:43:07,630 --> 00:43:10,620 reactions are that you use high temperatures -- remember 692 00:43:10,620 --> 00:43:12,050 we said the compromised temperature 693 00:43:12,050 --> 00:43:14,940 was 500 degrees celsius. 694 00:43:14,940 --> 00:43:17,800 We were talking about compressing the volume, so you 695 00:43:17,800 --> 00:43:21,030 want to put in energy to compress that volume. 696 00:43:21,030 --> 00:43:24,570 And so that is expensive. 697 00:43:24,570 --> 00:43:27,130 And you might want to think about a way, could you do that 698 00:43:27,130 --> 00:43:33,040 reaction at normal ambient temperature without applying 699 00:43:33,040 --> 00:43:34,730 energy to the system. 700 00:43:34,730 --> 00:43:39,780 Well, enzymes can do this reaction at ambient 701 00:43:39,780 --> 00:43:42,250 temperatures and they don't have to have high pressure 702 00:43:42,250 --> 00:43:45,810 associated with them, and the way the enzymes do this, and 703 00:43:45,810 --> 00:43:48,720 here's a little cartoon of a space-filling model of a 704 00:43:48,720 --> 00:43:52,090 particular enzyme called nitrogenase. 705 00:43:52,090 --> 00:43:56,910 So for nitrogen, and the ase means it's an enzyme. 706 00:43:56,910 --> 00:44:00,726 And the secret to this enzyme, how it can do what Haber and 707 00:44:00,726 --> 00:44:04,240 Bosch got two Nobel prizes for -- this enzyme has received no 708 00:44:04,240 --> 00:44:08,940 Nobel prizes, and it can do the same thing. 709 00:44:08,940 --> 00:44:11,420 Its secret are metals. 710 00:44:11,420 --> 00:44:15,500 It uses these clusters of metals in the context of the 711 00:44:15,500 --> 00:44:20,000 protein environment to do this chemistry, and they have iron, 712 00:44:20,000 --> 00:44:24,300 and they have also inorganic sulfide, and molybdenum, and 713 00:44:24,300 --> 00:44:29,230 so these combinations of metals can do this chemistry. 714 00:44:29,230 --> 00:44:31,890 And a number of scientists have been working for decades 715 00:44:31,890 --> 00:44:35,210 now trying to understand how the enzyme works. 716 00:44:35,210 --> 00:44:37,360 It's actually quite complicated and it's been 717 00:44:37,360 --> 00:44:39,140 pretty controversial. 718 00:44:39,140 --> 00:44:41,250 So let me just kind of show you at the heart of the 719 00:44:41,250 --> 00:44:46,410 enzyme, here all the atoms of the enzyme, but if you rotate 720 00:44:46,410 --> 00:44:50,530 the enzyme around and go in, the secret was that those 721 00:44:50,530 --> 00:44:52,230 combinations of metals. 722 00:44:52,230 --> 00:44:54,400 And one of the units that we're going to have second to 723 00:44:54,400 --> 00:44:58,330 last in this courses is about transition metals, that metals 724 00:44:58,330 --> 00:45:00,970 can do a lot of really important things in biological 725 00:45:00,970 --> 00:45:03,870 systems, and also in industrial processes and other 726 00:45:03,870 --> 00:45:06,290 things as well. 727 00:45:06,290 --> 00:45:10,130 So I do also like to mention research is 728 00:45:10,130 --> 00:45:11,410 going on here at MIT. 729 00:45:11,410 --> 00:45:16,250 I've mentioned Dick Schrock before, he won a Nobel Prize 730 00:45:16,250 --> 00:45:21,290 in chemistry a few years back, but not for his work on sort 731 00:45:21,290 --> 00:45:27,240 of nitrogenase systems, but he was able to design a small 732 00:45:27,240 --> 00:45:30,830 catalyst that can do this chemistry, that can catalyze 733 00:45:30,830 --> 00:45:34,160 this reduction of nitrogen to ammonia at a defined 734 00:45:34,160 --> 00:45:36,040 molybdenum center. 735 00:45:36,040 --> 00:45:38,050 Again, we'll talk about transition metals 736 00:45:38,050 --> 00:45:39,900 later in the course. 737 00:45:39,900 --> 00:45:43,440 And so his laboratory is very interested in coming up with 738 00:45:43,440 --> 00:45:49,280 better ways to split nitrogen than the industrial processes 739 00:45:49,280 --> 00:45:50,710 that are currently used. 740 00:45:50,710 --> 00:45:53,190 Some people are studying enzymes, other people are 741 00:45:53,190 --> 00:45:56,380 trying to use some of the tools that come out the 742 00:45:56,380 --> 00:46:00,460 enzymes, thinking about the enzymes and come up with other 743 00:46:00,460 --> 00:46:01,580 catalysts as well. 744 00:46:01,580 --> 00:46:04,510 So this work is going on here at MIT. 745 00:46:04,510 --> 00:46:11,520 All right, so let's give one more example of Le Chatelier 746 00:46:11,520 --> 00:46:15,980 from a biological perspective, and this has to do with the 747 00:46:15,980 --> 00:46:17,830 following reaction. 748 00:46:17,830 --> 00:46:23,230 The combination of oxygen with hemoglobin in your blood. 749 00:46:23,230 --> 00:46:27,670 And the reaction here, we have hemoglobin plus oxygen has 750 00:46:27,670 --> 00:46:33,970 oxyhemoglobin, so the oxygen is now bound to be hemoglobin. 751 00:46:33,970 --> 00:46:36,450 This reaction is very important for you. 752 00:46:36,450 --> 00:46:40,690 None of us would be alive if this was not happening. 753 00:46:40,690 --> 00:46:46,360 We need to get the oxygen from our lungs to our cells. 754 00:46:46,360 --> 00:46:49,590 So what happens if you decide to climb Mount 755 00:46:49,590 --> 00:46:51,040 Everest, for example. 756 00:46:51,040 --> 00:46:55,200 You may be thinking how hard can that be after a first 757 00:46:55,200 --> 00:46:57,270 semester freshman year at MIT. 758 00:46:57,270 --> 00:47:02,570 Really, I can think more broadly about it. 759 00:47:02,570 --> 00:47:03,970 All right, how many of you have climbed 760 00:47:03,970 --> 00:47:06,480 a mountain so far? 761 00:47:06,480 --> 00:47:07,910 Woah, a lot of people. 762 00:47:07,910 --> 00:47:10,630 OK, something about going to MIT and climbing big mountains 763 00:47:10,630 --> 00:47:12,030 seems to be connected. 764 00:47:12,030 --> 00:47:14,630 How many people feel like after surviving the first 765 00:47:14,630 --> 00:47:17,610 year, why not, go ahead and climb a mountain? 766 00:47:17,610 --> 00:47:18,830 Any more people? 767 00:47:18,830 --> 00:47:20,260 OK, no. 768 00:47:20,260 --> 00:47:24,510 So you gotta -- you've been through the hard part now, you 769 00:47:24,510 --> 00:47:26,250 don't want to risk anything anymore. 770 00:47:26,250 --> 00:47:29,140 All right, so this slide seems to be cutting off some stuff, 771 00:47:29,140 --> 00:47:31,120 so let's look over here. 772 00:47:31,120 --> 00:47:34,050 So what happens to this reaction? 773 00:47:34,050 --> 00:47:38,850 Well, as you go up higher and higher, the partial pressure 774 00:47:38,850 --> 00:47:40,720 of oxygen changes. 775 00:47:40,720 --> 00:47:47,980 So you're going to drop in partial pressure 0.2 to 0.14 . 776 00:47:47,980 --> 00:47:50,460 So what happens to this reaction 777 00:47:50,460 --> 00:47:54,210 if this value decreases? 778 00:47:54,210 --> 00:47:59,140 What happens, what direction does it shift? 779 00:47:59,140 --> 00:48:03,050 For products or reactants? 780 00:48:03,050 --> 00:48:03,830 Right. 781 00:48:03,830 --> 00:48:08,130 So, if this is going down, then some of your oxygenated 782 00:48:08,130 --> 00:48:12,140 hemoglobin is going to a release its oxygen. 783 00:48:12,140 --> 00:48:15,400 So it'll respond in such a way to compensate 784 00:48:15,400 --> 00:48:20,280 for that added stress. 785 00:48:20,280 --> 00:48:23,870 So what can you do to switch it back the other way? 786 00:48:23,870 --> 00:48:26,610 Again, we want our blood to be oxygenated. 787 00:48:26,610 --> 00:48:31,030 What happens to compensate for this if you're going to climb 788 00:48:31,030 --> 00:48:34,750 a mountain? 789 00:48:34,750 --> 00:48:37,300 What happens in the body? 790 00:48:37,300 --> 00:48:37,690 Yeah. 791 00:48:37,690 --> 00:48:41,080 STUDENT: The body can produce more hemoglobin. 792 00:48:41,080 --> 00:48:41,810 PROFESSOR: The body can produce more hemoglobin. 793 00:48:41,810 --> 00:48:45,320 So you often go to another altitude and hang out for a 794 00:48:45,320 --> 00:48:47,880 while before you start going way up. 795 00:48:47,880 --> 00:48:50,820 And what's happening during that time is your body 796 00:48:50,820 --> 00:48:53,940 realizes there is a problem, it knows about the Le 797 00:48:53,940 --> 00:48:57,110 Chatelier's principle, and so it's going to make more 798 00:48:57,110 --> 00:48:58,760 hemoglobin. 799 00:48:58,760 --> 00:49:02,750 And if you add more hemoglobin to this reaction, what 800 00:49:02,750 --> 00:49:06,600 direction does it go? 801 00:49:06,600 --> 00:49:10,450 Toward products, right. 802 00:49:10,450 --> 00:49:11,670 OK. 803 00:49:11,670 --> 00:49:18,060 So I wanted to mention significant figures, 804 00:49:18,060 --> 00:49:19,940 and there is -- 805 00:49:19,940 --> 00:49:23,130 I have to make a confession that when I first started 806 00:49:23,130 --> 00:49:26,570 teaching this course, I had never really paid attention to 807 00:49:26,570 --> 00:49:28,700 the rules of significant figures for logs. 808 00:49:28,700 --> 00:49:31,820 In this unit, the next unit, the unit after, you get the 809 00:49:31,820 --> 00:49:34,900 idea, there are going to be a lot of logs, and they have 810 00:49:34,900 --> 00:49:38,070 special rules for significant figures. 811 00:49:38,070 --> 00:49:40,590 So in the bottom of your handout, these rules are 812 00:49:40,590 --> 00:49:43,180 explained, it's also in your book. 813 00:49:43,180 --> 00:49:44,990 Pay attention to this. 814 00:49:44,990 --> 00:49:47,060 It's going to help you on all the 815 00:49:47,060 --> 00:49:49,610 problem-sets and in the course. 816 00:49:49,610 --> 00:49:54,030 So I just wanted to point out special rules about 817 00:49:54,030 --> 00:49:56,630 significant figures for logs. 818 00:49:56,630 --> 00:50:01,280 All right, that's it for today, see you on Wednesday.